1. Field of the Invention
The present invention relates generally to a servo control system and more particularly relates to a servo control system having a phase servo loop for an apparatus suitable to reproduce a signal recorded on a recording medium.
2. Description of the Prior Art
In general, upon reproducing a disc such as a pulse code modulated (hereinafter, simply referred to PCM) audio disc, a servo control system is required to rotate such disc stably with high precision. Whereas, in recording a PCM audio signal on a disc, two methods have been proposed. One method is to record the PCM audio signal on the disc at a constant angular velocity and the other method is to record the signal on the disc at a constant linear velocity. To increase a recording density, the recording method at the constant linear velocity is preferable. In this case, of course, the disc on which the signal is recorded at constant linear velocity must also be reproduced at the constant linear velocity. In one proposed method of controlling the disc rotation during reproduction, the reproduced signal from the disc is employed to control the disc rotation at the constant linear velocity.
The digitized (for instance, pulse code modulated) audio signal is generally recorded by the base band system which is not the carrier modulation method such as an amplitude modulation or the like.
When the signal is recorded by the base band system, a modulation method of a run length limited code is employed. In the run length limited code modulation method, with respect to data "0" or "1", a minimum transition interval T.sub.min of a transition between two data is lengthened to enhance efficiency of the recording, and a maximum transition interval T.sub.max therebetween is shortened to make self clocking on reproduction easier. The use of this modulation method allows the maximum or minimum transition interval T.sub.max or T.sub.min to become a predetermined value. Therefore, deviation of the maximum or minimum transition interval T.sub.max or T.sub.min from a reference value is detected and used as information to control the disc rotation at a predetermined linear velocity.
In this case, taking advantage of the fact that a modulation output in which the maximum transition interval T.sub.max occurs in succession will not normally occur, a bit pattern where the maximum transition interval T.sub.max occurs twice in sequence is employed as a frame synchronizing signal. Therefore, as this frame synchronizing signal always appears during one frame period, the servo control system is controlled to make the maximum transition interval T.sub.max equal to the reference value, so that the linear velocity can be made constant. The maximum transition interval T.sub.max of this case is selected to be 5.5T (where T represents the period of a bit cell of the input data). Accordingly, in the previously proposed method, T.sub.max being shorter than 5.5T forms a signal to decrease the rotational velocity of the motor or T.sub.max being longer than 5.5T forms a signal to increase the rotational velocity thereof. As a result, this makes the linear velocity of the disc constant. After the linear velocity of the disc was made constant, the servo control system is switched to a phase servo system by a change-over switch. This phase servo system includes such an arrangement that the frame synchronizing signal in the reproduced signal is phase compared with a signal of a frame period which is supplied from a reference oscillator such as a quartz oscillator and whose frequency is divided. Therefore, the motor is controlled in phase by an output of a phase comparator.
In the servo control system as described above, the reason why the phase servo circuit is not operable initially, but is made active through the change-over done by the switch after the linear velocity was made constant by the servo means is as follows. Since a PLL (phase locked loop) circuit used in a frame synchronizing signal detecting means is limited in lock range, unless the linear velocity has been locked constant beforehand by the lock means, the disc rotation can not be locked in phase to the output of the quartz oscillator following a large change of the linear velocity done by the scanning position of the pick-up device to reproduce the signal from the disc.
By the way, it may be considered that this servo control system is made as a perfectly digital circuit. In that case, the disc driving motor is generally driven by a pulse width modulated signal (hereinafter, referred to as PWM signal). Therefore, in consideration of the digitized servo control system, it is sufficient that the phase comparator in this phase servo system produces as its output the PWM signal corresponding to phase error. But, while the phase servo signal to be supplied to the above motor is the PWM signal with a repeating frequency of data frame, the frame frequency of PCM data to be recorded on the PCM audio disc is, for example, 7.35 KHz, which belongs to an audio frequency band. Thus the previously proposed servo control system has such a drawback that noise is caused by the motor driving signal.